Stent grafts are used to replace or repair vessels of the body such as arteries. A stent graft is usually formed from a tubular body of a biocompatible graft material with one or more stents mounted into or onto the tubular body to provide support therefore. The stents may be balloon expandable stents or self-expanding stents.
Endovascular methods have been proposed for treatment of aneurysms of the aorta particularly where the aneurysm is adjacent the aorta bifurcation. However, when an aneurysm occurs higher up in the aorta, in the region of the descending aorta adjacent the aortic or thoracic arch or in the ascending aorta, endovascular techniques for treating these aneurysms are somewhat more difficult because of the tight curvature of the aortic or thoracic arch, the occurrence of major arteries in the region and the proximity to the heart. Placement of a substantially cylindrical prosthesis in such a curved region can cause problems.
Stent grafts are typically deployed using endovascular techniques on an introduction device in which the stent graft is retained in a radially contracted condition by a sheath. Upon withdrawal of the sheath and release of any retention arrangement where provided, for example in cases in which the stent graft has self-expanding stents, the stent graft can expand under the action of the self-expanding stents towards the vessel walls to redefine the blood flow path. The introduction device is withdrawn after deployment.
Currently, stent grafts are deployed in curved lumens by causing these to follow the curvature imparted to the introducer. However, this can result in the stent graft not sitting properly in the blood vessel and in the lumen of the prosthesis being closed off or reduced in lumen diameter.
Furthermore, when deploying a stent graft that is substantially straight in a curved aorta there is a danger that the proximal end of the stent graft, that is, the end nearest the heart, will not lie flat against the walls of the aorta (i.e., the end “face” is not positioned perpendicularly to the wall of the vessel) and blood can flow underneath the edge of the graft, particularly on the inner side of the curve of the aortic or thoracic arch and cause the stent graft to buckle and close off thereby causing serious problems.
FIGS. 1 and 2 illustrate this problem. The introducer and stent graft generally have a substantially straight configuration but as a result of their pliancy are urged into a curved orientation by the walls of the lumen. However, as the stent graft is held on the introducer, which itself tends to maintain its straightened configuration to the extent that it keeps to the outside of the curve of the lumen, the stent graft also tends to maintain a lesser curved configuration as it is being deployed. When the stent graft is released from its ties to the introducer, it expands in a manner which continues to tend to the straightened configuration and in particular to push the inner edge of the stent graft forwardly. This leads to an increased Proximal Face Angle (PFA), that is, to an increased angle between the line formed by the proximal end of the stent graft and the line perpendicular to the walls of the lumen. It also leads to an increased Proximal Normal Gap (PNG), that is, a gap between the lumen wall and the proximal end of the stent on the inner part of the bend in the lumen. These are shown in FIGS. 1 and 2. The Proximal Normal Gap allows blood pressure to build between the outside of the stent graft and the lumen wall, which will tend to bias this side of the stent graft inwardly into the lumen and thus towards closing of the lumen of the stent graft. Thus, the seal at the proximal end of the stent graft may not be as effective as desired. This gap can also cause the graft material itself to flap in the fluid flow, leading to unnatural fluid flow and possible premature wear and tear of the stent graft.
US 2004/0073289 discloses a stent graft for deployment within a curved portion of the aorta.